Superhydrous aluminous silica phases as major water hosts in high-temperature lower mantle

Abstract

Water transported by subducted oceanic plates changes mineral and rock properties at high pressures and temperatures, affecting the dynamics and evolution of the Earth’s interior. Although geochemical observations imply that water should be stored in the lower mantle, the limited amounts of water incorporation in pyrolitic lower-mantle minerals suggest that water in the lower mantle may be stored in the basaltic fragments of subducted slabs. Here, we performed multianvil experiments to investigate the stability and water solubility of aluminous stishovite and CaCl 2 -structured silica, referred to as poststishovite, in the SiO 2 -Al 2 O 3 -H 2 O systems at 24 to 28 GPa and 1,000 to 2,000 °C, representing the pressure–temperature conditions of cold subducting slabs to hot upwelling plumes in the top lower mantle. The results indicate that both alumina and water contents in these silica minerals increase with increasing temperature under hydrous conditions due to the strong Al 3+ -H + charge coupling substitution, resulting in the storage of water up to 1.1 wt %. The increase of water solubility in these hydrous aluminous silica phases at high temperatures is opposite of that of other nominally anhydrous minerals and of the stability of the hydrous minerals. This feature prevents the releasing of water from the subducting slabs and enhances the transport water into the deep lower mantle, allowing significant amounts of water storage in the high-temperature lower mantle and circulating water between the upper mantle and the lower mantle through subduction and plume upwelling. The shallower depths of midmantle seismic scatterers than expected from the pure SiO 2 stishovite–poststishovite transition pressure support this scenario.

Temperature Dependence of H2O Solubility in Al‐Free Stishovite

“…Within the mutual uncertainties, single‐crystal XRD measurements revealed identical unit‐cell parameters of all samples. No distortion to post‐stishovite was observed, unlike in hydrous Al‐bearing samples recovered from similar pressure‐temperature conditions (Ishii, Criniti, et al., 2022). Single‐crystal XRD also revealed that the crystals from each run exhibited very sharp and narrow peaks, with full width half maxima of <0.06° measured on omega scans, confirming the high degree of crystallinity of the stishovite samples.…”

“…Recently, it was found that Al‐bearing silica in CaCl 2 ‐structured stishovite, i.e., post‐stishovite, can contain up to ∼11,000 wt. ppm at 24–28 GPa and 1000–2000°C (Ishii, Criniti, et al., 2022), corresponding to the topmost lower mantle conditions. Therefore, we can still consider Al‐bearing stishovite as a potential water carrier in the topmost lower mantle.…”

NanoSIMS analysis of water content in bridgmanite at the micron scale: An experimental approach to probe water in Earth’s deep mantle

“…Although dissolved in mantle minerals in trace amounts, water can greatly influence their phase relations, transport, and elastic properties (e.g., Inoue, 1994;Du et al, 2019;Freitas and Manthilake, 2019;Zhang et al, 2022). Despite much progress on how water may be dissolved in upper mantle and transition zone minerals (e.g., Hauri et al, 2006;Inoue et al, 2010;Pearson et al, 2014;Gu et al, 2022), its quantity and chemical properties throughout lower mantle are largely unconstrained (e.g., Litasov et al, 2003;Fu et al, 2019;Lin et al, 2020;Ishii et al, 2022a). Since Bridgmanite (Brg) is the most abundant mineral in silicate mantle (Figures 1A,B, e.g., Hirose et al, 2017;Irifune and Tsuchiya., 2015;Marquardt and Thomson., 2020), its water content would provide critical constraints on how water is distributed and cycled within the Earth.…”

Pressure stabilizes ferrous iron in bridgmanite under hydrous deep lower mantle conditions

“…Bridgmanite is nearly dry in coexistence with a hydrous phase 24 . Water can be stored in the high-pressure phases of silica in a basaltic composition 23 , 30 – 32 . Under the deep lower mantle conditions, the Al 3+ -rich NiAs-type silica phase with an approximate formula Si 0.7 Al 0.3 O 1.85 H x 23 could contain up to 4.6 wt.% H 2 O via the Si 4+ = Al 3+ + H + charge-coupled substitution 30 , 33 .…”